cleaned up cruft from support core copy

Author: sstadelman

swift-tools-support-core/Package.swift

@@ -15,7 +15,6 @@ import PackageDescription
 
 let package = Package(
     name: "swift-tools-support-core",
-    platforms: [.macOS(.v10_13)],
     products: [
         .library(
             name: "SwiftToolsSupport",